The invention relates to a circuit arrangement for enhancing the sharpness of edges in a video signal, in particular a chroma signal or a color-difference signal, in which an input of the circuit arrangement is coupled to an output of the circuit arrangement by means of at least one switch and in which the input is coupled to a switching-signal generator for the generation of a switching signal for the actuation of the switch, so that during the occurrence of edges, the coupling between the input and the output is interrupted by means of the switch, the circuit arrangement comprising a memory for the amplitude of the video signal at the beginning of an edge.
When a signal with a steplike transition between two amplitude values is applied to a color-signal transmission channel, the limited bandwidth gives rise to a transition with a certain slope. This results in a transition interval between the initial value and the final value, which interval has a specific width which is largely independent of the magnitude of the amplitude step. This width depends on the frequency bandwidth of the color-signal transmission channel and may therefore have different values, for example, in different types of TV-sets.
In a circuit arrangement of the type mentioned in the opening paragraph as disclosed in DE-AS 15 62 170, a switching signal is derived from edges of a video signal, by means of which switching signal, it is achieved that upon the appearance of a sloping transition between two video-signal values, the signal value existing before the transition interval is stored directly and after a delay of half the width of the transition interval, the signal value which occurs after the transition interval is transferred to the memory and, via a switch arranged before the output terminal, to the output. In the known circuit arrangement the insertion of the signal values with steep transitions in the range in which the incoming signal has only a finite slope owing to the limited frequency bandwidth, is started by a pulse signal which occurs at the beginning of the transition interval and is controlled by means of delay lines.
The known circuit arrangement requires the use of a plurality of switches and, in particular, delay lines, which must be proportioned in conformity with the width of the transition interval. Since the interval width depends on the frequency bandwidth, which in its turn depends on the design of the entire signal transmission channel, the delays of the delay means used must be variable or adjustable. Therefore, the known circuit arrangement is either intricate or a compromise, which cannot yield optimum results.